Method for making a lithographic printing plate

ABSTRACT

The present invention provides a method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of: 
     image-wise exposing an imaging element comprising on a support a photosensitive layer comprising a silver halide emulsion and an image receiving layer containing physical development nuclei, 
     developing said imaging element in the presence of at least one developing agent and at least one silver halide solvent using an alkaline processing liquid and 
     neutralizing a thus obtained developed imaging element using a neutralization solution having a pH between 4 and 10.5 and comprising a buffer characterized in that said neutralization solution comprises 2-methyl-3(2H)-isothiazolone.

Priority is claimed under 35 USC 119(E) from provisional application Ser. No. 60/026671 filed Sep. 25, 1996.

1. Field of the Invention.

The present invention relates to a method for making a lithographic printing plate according to the silver salt diffusion transfer process.

2. Background of the Invention.

The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde - The Focal Press - London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the imaging element.

A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.

The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.

As for other printing plates it is required that the printing plates obtained according to the DTR-process have a high printing endurance, good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). It is furthermore desirable, especially for low printing runs, that the number of copies that have to be disposed of because of poor ink acceptance in the printing areas and/or ink acceptance in the non-printing areas during start-up of the printing process is limited. Besides the above requirements the printing plate preferably exhibits a high resolving power.

In order to obtain these required good printing properties with a plate obtained according to the DTR-process, a number of treatments of a developed mono-sheet DTR-material have been proposed. In EP-A 590,204 it has been proposed to treat a developed monosheet DTR-material with a neutralization solution having a pH less than 7 and containing a meso-ionic compound. A reduction in the number of copies that have to be disposed of and an increase of the resolution was found. However, the meso-ionic compounds are expensive and the achieved result is still unsatisfactory.

Neutralization solutions having a pH around 5.6 possibly containing cysteine have also been used to obtain the objective of decreasing the number of useless copies during startup. EP-A 665,466 discloses the treatment of a developed monosheet DTR-material with a neutralization solution having a pH between 5 and 7 and containing a total concentration of phosphate ions between 0.2 mole/l and 0.4 mole/l. EP-A 95200555.1 discloses the treatment of a developed monosheet DTR-material with a neutralization solution having a pH between 5 and 10.5 and containing specified amines.

Although such neutralization solutions are all to a certain degree effective for decreasing the number of useless copies during startup, they have a inherent low capacity and need to be refreshed frequently. Indeed, when said neutralization solutions are used to treat a certain amount of developed imaging elements over a longer period of time the then obtained printing plates show a high number of useless copies during startup due to a poor ink acceptance. For economical and ecological reasons and for convenience it is desirable that the capacity of these neutralization solutions is improved.

3. Summary of the Invention.

It is an object of the present invention to provide an economical, ecological and convenient method for making a lithographic printing plate according to the DTR-process having good printing properties i.e. good ink acceptance in the printing areas, no ink acceptance in the non-printing areas and a limited number of useless copies during startup.

Further objects of the present invention will become clear from the description hereinafter.

According to the present invention there is provided a method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of:

image-wise exposing an imaging element comprising on a support a photosensitive layer comprising a silver halide emulsion and an image receiving layer containing physical development nuclei,

developing said imaging element in the presence of at least one developing agent and at least one silver halide solvent using an alkaline processing liquid and

neutralizing a thus obtained developed imaging element using a neutralization solution having a pH between 4 and 10.5 and comprising a buffer characterized in that said neutralization solution comprises 2-methyl-3(2H)-isothiazolone.

4. Detailed Description of the Invention.

It has been found that with a lithographic printing plate prepared according to the DTR-process and neutralized with a neutralization solution having a pH between 4 and 10.5 and comprising a buffer and 2-methyl-3(2H)-isothiazolone, the number of copies that have to be disposed of during start-up of the printing process remains limited even when a sizable amount of developed imaging elements has been treated by said neutralization solution. However the lifetime of the neutralization solution is severely restricted when it contains no 2-methyl-3(2H)-isothiazolone.

The structure of 2-methyl-3(2H)-isothiazolone is graphically represented by formula I ##STR1##

The amount of said 2-methyl-3(2H)-isothiazolone in a neutralization solution preferably is at least 1 mg/l, more preferably at least 5 mg/l and most preferably at least 8 mg/l. The upper limit is not so important but is for practical reasons preferably not higher than 100 mg/l, more preferably not higher than 50 mg/l, most preferably not higher than 25 mg/l.

The neutralization solution comprises a buffer, preferably a phosphate or a citrate buffer, more preferably a monosodium and/or monopotassium dihydrogen phosphate, most preferably in an amount between 20 g/l and 80 g/l.

The pH at 25° C. of a fresh neutralization solution lies preferably between 4 and 9, more preferably between 5 and 8, but may increase during use to a value of about 10.5. The pH of a fresh neutralization solution may be adjusted, if necessary, by adding to the aqueous solution according to the present invention an organic or preferably an inorganic acid or inorganic alkali.

According to the present invention the neutralizing solution preferably also contains a hydrophobizing agent for improving the hydrophobicity of the silver image obtained in the image receiving layer. The hydrophobizing agents used in connection with the present invention are compounds that are capable of reacting with silver or silver ions and that are hydrophobic i.e. insoluble in water or only slightly soluble in water. Generally these compounds contain a mercapto group or thiolate group and one or more hydrophobic substituents e.g. an alkyl group containing at least 3 carbon atoms. Preferred hydrophobizing agents are mercapto-1,3,4-thiadiazoles as described in DE-A 1,228,927 and in U.S. Pat. No. 4,563,410, 2-mercapto-5-alkyl-oxa-3,4-diazole, 3-mercapto-1,2,4-triazoles and aryl substituted mercaptotetrazoles.

Particularly preferred hydrophobizing agents in connection with the present invention are at least 5 carbon atoms comprising aliphatic chain substituted mercaptotetrazoles, more in particular those according to the following formula (II): ##STR2## wherein Z represents an alkylaryl, a substituted or unsubstituted alkenyl, a substituted or unsubstituted alkynyl and most preferably a substituted or unsubstituted alkyl.

Specific examples of hydrophobizing agents according to formula (II) are listed in table 1.

                  TABLE 1                                                          ______________________________________                                         compound no.       Z in formula (II)                                           ______________________________________                                         1                  CH.sub.2 CONH-n.C.sub.6 H.sub.13                            2                  CH.sub.2 CON(n.C.sub.4 H.sub.9).sub.2                       3                  n.C.sub.6 H.sub.13                                          ______________________________________                                    

According to the present invention the hydrophobizing agents are contained in the neutralizing solution in an amount of at least 0.1 g/l, more preferably at least 0.2 g/l and most preferably at least 0.3 g/l. The maximum amount of hydrophobizing agents will be determined by the type of hydrophobizing agent etc. Typically the concentration of hydrophobizing agent is preferably not more than 1.5 g/l and more preferably not more than 1 g/l.

The neutralization solution according to the present invention preferably also contains a sequestering compound, more preferably phosphonic acid, (1-hydroxyethylidene) bis- (sold under the name of TURPINAL SL by Henkel, Belgium and as DEQUEST 2010 by Monsanto, Belgium) or its disodium salt (sold under the name of TURPINAL 2NZ by Henkel, Belgium). The amount of said sequestering agent is preferably comprised between 0.100 and 10 g/l.

Preferably said neutralizing solution comprises an agent having antioxidation activity e.g. sodium benzoate. The amount of said agent having antioxidation activity is preferably comprised between 0.100 and 10 g/l.

Preferably said neutralizing solution comprises an amine. Said amine preferably corresponds to the following formula: NR¹ R² R³ wherein R¹ represents a C₁ -C₁₂ hydrocarbon group containing a hydrophilic group and R² and R³ each independently represents a hydrogen, a C₁ -C₁₂ hydrocarbon group or R² and R³ together with the atoms to which they are attached complete a 5- or 6- membered ring, with the proviso that at least 1 of the groups represented by R² and R³ is hydrogen or an aliphatic group.

Said C₁ -C₁₂ hydrocarbon groups include unsubstituted or substituted C₁ -C₁₂ alkyl groups, unsubstituted or substituted C₁ -C₁₂ alkenyl groups, unsubstituted or substituted C₁ -C₁₂ alkynyl groups, unsubstituted or substituted C₅ -C₁₂ cycloalkyl groups, each of which may contain one or more divalent linking group such as --O--, --S--, etc., unsubstituted or substituted C₇ -C₁₂ aralkyl groups, unsubstituted or substituted C₆ -C₁₂ aryl groups and unsubstituted or substituted C₄ -C₁₂ heterocyclic groups, said C₁ -C₁₂ hydrocarbon group being substituted by at least a hydrophilic group when representing R¹.

Said hydrophilic group can be an amino group, an ammonium group, a hydroxyl, a sulfo group or a carboxyl.

Said compounds can be prepared by well known methods e.g. reaction of ammonia or an amine with the appropriate hydrocarbon halide, sulfonate, oxyde, etc.

More preferably, R¹ represents a C₁ -C₁₂ alkyl group substituted by a hydrophilic group and R² and R³ each independently represents a hydrogen or an unsubstituted or substituted C₁ -C₁₂ alkyl group. Still more preferably, R¹ represents a C₁ -C₈ alkyl group substituted by a hydrophilic group and R² and R³ each independently represents a hydrogen, an unsubstituted C₁ -C₅ alkyl group or a C₁ -C₅ alkyl group substituted by a hydrophilic group.

The ratio in said compounds between the number of hydrophilic groups to the number of carbon atoms, excluding the carbon atoms of the carboxyls is preferably at least 0.1, more preferably at least 0.2, most preferably at least 0.3.

Preferably at least 1 of the hydrophilic groups is a hydroxyl. More preferably hydroxyls account for at least 20% by number, still more preferably for at least 40% by number, most preferably for at least 50% by number of the hydrophilic groups.

Very preferred compounds are alkanolamines which may be of the tertiary, secundary or primary type and which correspond to the following formula: ##STR3## wherein X and X' independently represent hydrogen, a hydroxyl group or an amino group, 1 and m represent 0 or integers of 1 or more and n represents an integer of 1 or more. Preferably used alkanolamines are e.g. N-(2-aminoethyl)ethanolamine, diethanolamine, N-methylethanolamine, triethanolamine, N-ethyldiethanolamine, diisopropanolamine, ethanolamine, 4-aminobutanol, N,N-dimethylethanolamine, 3-aminopropanol, N,N-ethyl-2,2'-iminodiethanol etc. or mixtures thereof. Most preferably said amines are no silver solvent like triethanolamine.

Said amines can be used alone or in combination with each other. The total concentration of said compounds in the neutralization solution is preferably comprised between 0.01 mole/l and 0.5 mole/l, more preferably between 0.05 mole/l and 0.1 mole/l.

The neutralizing solution may contain substances which influence the hydrophobic/hydrophilic balance of the printing plate obtained after processing of the DTR element, e.g. silica. Further the neutralization solution can contain wetting agents, preferably compounds containing perfluorinated alkyl groups.

The neutralization solution may contain organic solvents in a small amount for increasing the solubility of hydrophobic compounds.

The concentrations given above are for a neutralization solution ready-for-use. The components of said neutralization solution can also be used in a solid form or preferably said neutralization solutions is prepared in a concentrated form. In a very preferred form the concentrated neutralization solution has a concentration of the various ingredients which is a twentyfold of the concentration of these ingredients in a ready-to-use neutralization solution. To prepare a ready-to-use neutralization solution from said concentrated neutralization solution one part by volume is diluted with 19 parts by volume of water.

The alkaline processing liquid used for developing the imaging element in accordance with the method of the present invention preferably contains a silver halide solvent. Preferably the silver halide solvent is used in an amount between 0.01% by weight and 10% by weight and more preferably between 0.05% by weight and 8% by weight. Suitable silver halide solvents for use in connection with the present invention are e.g. 2-mercaptobenzoic acid, cyclic imides, oxazolidones and thiosulfates. Silver halide solvents that are preferably used in connection with the present invention are thiocyanates and alkanolamines as described above.

According to the present invention the alkanolamines are preferably present in the alkaline processing liquid. However part or all of the alkanolamine can be present in one or more layers of the imaging element.

A further suitable type of silver halide solvents are thioether compounds. Preferably used thioethers correspond to the following general formula:

    Z--(R.sup.1 --S).sub.t --R.sup.2 --S--R.sup.3 --Y

wherein Z and Y each independently represents hydrogen, an alkyl group, an amino group, an ammonium group, a hydroxyl, a sulfo group, a carboxyl, an aminocarbonyl or an aminosulfonyl, R¹, R² and R³ each independently represents an alkylene that may be substituted and optionally contains an oxygen bridge and t represents an integer from 0 to 10. Examples of thioether compounds corresponding to the above formula are disclosed in e.g. U.S. Pat. No. -4,960,683 and EP-A-554.585.

Still further suitable silver halide solvents are meso-ionic compounds. Preferred meso-ionic compounds for use in connection with the present invention are triazolium thiolates and more preferred 1,2,4-triazolium-3-thiolates.

According to a preferred embodiment of the present invention at least part and most preferably all of the meso-ionic compound is present in the alkaline processing liquid used for developing the image-wise exposed imaging element. Preferably the amount of meso-ionic compound in the alkaline processing liquid is between 0.1mmol/l and 25 mmol/l and more preferably between 0.5 mmol/l and 15 mmol/l and most preferably between 1 mmol/l and 8 mmol/l.

However the meso-ionic compound may be incorporated in one or more layers comprised on the support of the imaging element. The meso-ionic compound is in that case preferably contained in the imaging element in a total amount between 0.1 and 10 mmol/m², more preferably between 0.1 and 5 mmol/m² and most preferably between 0.5 and 1.5 mmol/m². More details are disclosed in EP-A-0,554,585

According to the present invention the alkaline processing liquid preferably also contains hydrophobizing agents as described above for improving the hydrophobicity of the silver image obtained in the image receiving layer. Particularly preferred compounds are 5-n-heptyl-2-mercapto-1,3,4,-oxadiazol and 3-mercapto-4-acetamido-5-n-heptyl-1,2,4-triazole.

The hydrophobizing agents are contained in the alkaline processing solution in an amount of preferably at least 0.1 g/l. more preferably at least 0.2 g/l and most preferably at least 0.3 g/l. The maximum amount of hydrophobizing agents will be determined by the type of hydrophobizing agent, type and amount of silver halide solvents etc. Typically the concentration of hydrophobizing agent is preferably not more than 1.5 g/l and more preferably not more than 1 g/l.

The alkaline processing liquid used in accordance with the present invention preferably has a pH between 9 and 14 and more preferably between 10 and 13. Said pH may be established by an organic or inorganic alkaline substance or a combination thereof. Suitable inorganic alkaline substances are e.g. potassium or sodium hydroxide, carbonate, phosphate etc. Suitable organic alkaline substances are e.g. alkanolamines. In the latter case the alkanolamines will provide or help maintain the pH and serve as a silver halide complexing agent.

The alkaline processing liquid may also contain the developing agent(s) used in accordance with the present invention. In this case the alkaline processing liquid is called a developer. On the other hand some or all of the developing agent(s) may be present in one or more layers of the imaging element. When all of the developing agents are contained in the imaging element the alkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the present invention are preferably of the p-dihydroxybenzene type, e.g. hydroquinone, methylhydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a 1-phenyl-3-pyrazolidone-type developing agent and/or p-monomethylaminophenol. Particularly useful auxiliary developing agents are of the phenidone type e.g. 1-phenyl-3-pyrazolidone, 1-phenyl-4-monomethyl-3-pyrazolidone, and 1-phenyl-4,4-dimethyl-3-pyrazolidone. However other developing agents can be used.

The alkaline processing liquid preferably also contains a preserving agent having antioxidation activity, e.g. sulphite ions provided e.g. by sodium or potassium sulphite. For example, the aqueous alkaline solution comprises sodium sulphite in an amount ranging from 0.15 to 1.0 mol/l. Further may be present a thickening agent, e.g. hydroxyethylcellulose and carboxymethylcellulose, fog inhibiting agents, e.g. potassium bromide, potassium iodide and a benzotriazole which is known to improve the printing endurance, calcium-sequestering compounds, anti-sludge agents, and hardeners including latent hardeners.

Development acceleration can be accomplished with the aid of various compounds to the alkaline processing liquid and/or one or more layers of the imaging element, preferably polyalkylene derivatives having a molecular weight of at least 400 such as those described in e.g. U.S. Pat. Nos. 3,038,805 - 4,038,075 - 4,292,400 - 4,975,354.

According to the present invention the imaging element can be information-wise exposed in an apparatus according to its particular application, e.g. a conventional process camera containing a conventional light source or a laser containing device.

The imaging element in connection with the present invention for preparing a lithographic printing plate essentially contains on a support in the order given a photosensitive layer containing silver halide and an image receiving layer containing physical development nuclei in water permeable relationship with said emulsion layer.

Layers being in waterpermeable contact with each other are layers that are contiguous to each other or only separated from each other by (a) waterpermeable layer(s). The nature of a waterpermeable layer is such that it does not substantially inhibit or restrain the diffusion of water or of compounds contained in an aqueous solution e.g. developing agents or complexed silver ions.

Supports suitable for use in accordance with the present invention may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer, e.g. a polyethylene layer which optionally contains an anti-halation dye or pigment. It is also possible to use an organic resin support e.g. cellulose nitrate film, cellulose acetate film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film, polyvinylchloride film or poly-Alpha-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a hydrophilic adhesion layer which can contain water insoluble particles such as silica or titanium dioxide. Metal supports e.g. aluminium may also be used in accordance with the present invention.

The image receiving layer containing physical development nuclei used in accordance with the present invention can be free of hydrophilic binder but comprises preferably small amounts upto 80% by weight of the total weight of said layer of a hydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicity of the surface. Preferred development nuclei for use in accordance with the present invention are sulphides of heavy metals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, and zinc. Especially suitable development nuclei in connection with the present invention are palladium sulphide nuclei. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans, and tin (II) halides. Heavy metals, preferably silver, gold, platinum, palladium, and mercury can be used in colloidal form.

For use in connection with the present invention the silver halide emulsion or emulsions preferably consist principally of silver chloride while a fraction of silver bromide may be present ranging from 0 mole % to 40 mole %. Most preferably a silver halide emulsion containing at least 70 mole % of silver chloride is used. The emulsions containing more than 5 mole % of silver bromide preferably belong to the core/shell type well known to those skilled in the art in the sense that substantially all the bromide is concentrated in the core. This core contains preferably 10 to 40% of the total silver halide precipitated, while the shell consists preferably of 60 to 90% of the total silver halide precipitated.

The photographic silver halide emulsion(s) used in accordance with the present invention can be prepared from soluble silver salts and soluble halides according to different methods as described e.g. by P. Glafkides in "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G.F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966), and by V. L. Zelikman et al. in "Making and Coating Photographic Emulsion", The Focal Press, London (1966).

The average size of the silver halide grains may range from 0.10 to 0.70 μm , preferably from 0.25 to 0.45 μm.

Preferably during the precipitation stage Iridium and/or Rhodium containing compounds or a mixture of both are added. The concentration of these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole of AgNO₃, preferably between 0.5*10⁻⁷ and 10⁻⁵ mole per mole of AgNO₃.

The emulsions can be chemically sensitized e.g. by adding sulphur-, seleno- and/or telluro-containing compounds during the chemical ripening stage e.g. allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Also reducing agents e.g. the tin compounds described in BE-P 493,464 and 568,687, and polyamines such as diethylene triamine or derivatives of aminomethane-sulphonic acid can be used as chemical sensitizers. Other suitable chemical sensitizers are noble metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium. This method of chemical sensitization has been described in the article of R. KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

The emulsions of the DTR element can be spectrally sensitized according to the spectral emission of the exposure source for which the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region include methine dyes such as those described by F. M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used for this purpose include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine dyes, complex merocyanine dyes.

In the case of a conventional light source, e.g. tungsten light or a green laser, a green sensitizing dye is needed. In case of exposure by an argon ion laser a blue sensizing dye is incorporated. In case of exposure by a red light emitting source, e.g. a LED or a HeNe laser a red sensitizing dye is used. In case of exposure by a semiconductor laser special spectral sensitizing dyes suited for the near infra-red are required. Suitable infra-red sensitizing dyes are disclosed in i.a. U.S. Pat. Nos. 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, 3,623,881 and 3,695,888.

A preferred blue sensitizing dye, green sensitizing dye, red sensitizing dye and infra-red sensitizing dye in connection with the present invention are described in EP-A 554,585.

To enhance the sensitivity in the red or near infra-red region use can be made of so-called supersensitizers in combination with red or infra-red sensitizing dyes. Suitable supersensitizers are described in Research Disclosure Vol 289, May 1988, item 28952. The spectral sensitizers can be added to the photographic emulsions in the form of an aqueous solution, a solution in an organic solvent or in the form of a dispersion.

The silver halide emulsions may contain the usual emulsion stabilizers. Suitable stabilizers are azaindenes, preferably tetra-or penta-azaindenes, especially those substituted with hydroxy or amino groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitable stabilizers are i-a. heterocyclic mercapto compounds e.g. phenylmercaptotetrazole, quaternary benzothiazole derivatives, and benzotriazole. Preferred compounds are mercapto substituted pyrimidine derivatives as disclosed in U.S. Pat. No. 3,692,527.

The silver halide emulsions may contain pH controlling ingredients. Preferably the emulsion layer is coated at a pH value near the isoelectric point of the gelatin to improve the stability characteristics of the coated layer. Other ingredients such as antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin may be present. The silver halide emulsion layer may comprise light-screening dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787, DE-P 2,453,217.

More details about the composition, preparation and coating of silver halide emulsions can be found in e.g. Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109.

In addition to the above described emulsion layer and image receiving layer other hydrophilic colloid layers in water permeable relationship with these layers may be present. For example it is especially advantageous to include a base-layer between the support and the photosensitive silver halide emulsion layer. In a preferred embodiment of the present invention said base-layer serves as an antihalation layer. This layer can therefore contain the same light-absorbing dyes as described above for the emulsion layer; as alternative finely divided carbon black can be used for the same antihalation purposes as described in U.S. Pat. No. 2,327,828. On the other hand, in order to gain sensitivety, light reflecting pigments, e.g. titaniumdioxide can be present. Further this layer can contain hardening agents, matting agents, e.g. silica particles, and wetting agents. At least part of these matting agents and/or light reflection pigments may also be present in the silver halide emulsion layer the most part however preferably being present in said base-layer. As a further alternative the light reflecting pigments may be present in a separate layer provided between the antihalation layer and the photosensitive silver halide emulsion layer.

In a preferred embodiment in connection with the present invention a backing layer is provided at the non-light sensitive side of the support. This layer which can serve as anti-curl layer can contain i.a. matting agents e.g. silica particles, lubricants, antistatic agents, light absorbing dyes, opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and wetting agents. The backing layer can consist of one single layer or a double layer pack.

The hydrophilic layers usually contain gelatin as hydrophilic colloid binder. Mixtures of different gelatins with different viscosities can be used to adjust the Theological properties of the layer. Like the emulsion layer the other hydrophilic layers are coated preferably at a pH value near the isoelectric point of the gelatin. But instead of or together with gelatin, use can be made of one or more other natural and/or synthetic hydrophilic colloids, e.g. albumin, casein, zein, polyvinyl alcohol, alginic acids or salts thereof, cellulose derivatives such as carboxymethyl cellulose, modified gelatin, e.g. phthaloyl gelatin etc.

The hydrophilic layers of the imaging element, especially when the binder used is gelatin, can be hardened with appropriate hardening agents such as those of the vinylsulfone type e.g. methylenebis(sulfonylethylene), aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts of the type, described in U.S. Pat. No. 4,063,952.

Preferably used hardening agents are of the aldehyde type. The hardening agents can be used in wide concentration range but are preferably used in an amount of 4% to 7% of the hydrophilic colloid. Different amounts of hardener can be used in the different layers of the imaging element or the hardening of one layer may be adjusted by the diffusion of a hardener from another layer.

The imaging element used according to the present invention may further comprise various kinds of surface-active agents in the photographic emulsion layer or in at least one other hydrophilic colloid layer. Examples of suitable surface-active agents are described in e.g. EP 545452. Preferably compounds containing perfluorinated alkyl groups are used.

The imaging element of the present invention may further comprise various other additives such as e.g. compounds improving the dimensional stability of the imaging element, UV-absorbers, spacing agents and plasticizers.

Suitable additives for improving the dimensional stability of the imaging element are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl (meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth)acrylates, and styrene sulphonic acids.

The present invention will now be illustrated by the following example without however limiting it thereto. All parts are by weight unless otherwise specified.

EXAMPLE

Preparation of the silver halide emulsion coating solution.

A silver chlorobromide emulsion composed of 98.2 mol % of chloride and 1.8 mol % of bromide was prepared by the double jet precipitation method. The average silver halide grain size was 0.4 μm (diameter of a sphere with equivalent volume) and contained Rhodium ions as internal dopant. The emulsion was orthochromatically sensitized and stabilized by 1-phenyl-5-mercapto-tetrazole.

A base layer coating solution was prepared having the following composition:

    ______________________________________                                         gelatin           5.5%                                                         carbon black      0.76%                                                        silica particles (5 μm)                                                                       1.6%                                                         ______________________________________                                    

Preparation of the imaging element.

The emulsion coating solution and base layer coating solution were simultaneously coated by means of the cascade coating technique to a polyethylene terephthalate support provided with a pack of two backing layers such that the base layer coating was coated directly to the side of the support opposite to the side containing said backing layers. The emulsion layer was coated such that the silver halide coverage expressed as AgNO₃ was 1.5 g/m² and the gelatin content was 1.5 g/m². The emulsion layer further contained 0.15 g/m² of 1-phenyl-4,4'-dimethyl-3-pyrazolidone and 0.25 g/m² of hydroquinone. The base layer was coated such that the amount of gelatin in the coated layer was 3 g/m².

The first subbing layer on the backside comprised 3.1 mg/m² of an antistatic composition consisting of 2.2 mg/m² of polystyrene sulphonic acid and 0.9 mg/m² of poly(3,4-ethylenedioxy-thiophene).

The layer nearest to the support of the backing layer pack contained 0.08 g/m² of gelatin. The second backing layer contained 2.8 g/m² of gelatin, 0.050 g/m² of a matting agent consisting of transparent spherical polymeric beads of 3 micron average diameter according to EP-A-080,225, 1.5 g/m² of dispersed barium sulfate (average diameter 0.8-1.1 mμ), 1.0 g/m² of colloidal silica, 0.05 g/m² of hardening agent triacrylformal, 9 mg/m² of non-perfluoro surfactants and 0.021 g/m² of wetting agent F₁₅ C₇ --COONH₄.

The thus obtained element was dried and subjected to a temperature of 40° C. for 5 days and then the emulsion layer was overcoated with a layer containing PdS as physical development nuclei at 0.7 mg/m², hydroquinone at 0.4 g/m² and formaldehyde at 100 mg/m².

Finally the imaging element was cut in sheets.

The following processing solutions were prepared:

    ______________________________________                                         Activator                                                                      ______________________________________                                         sodium hydroxide       30     g                                                sodium sulphite anh.   35     g                                                2-aminoethyl-aminoethanol                                                                             20     ml                                               1-methyl, 4-allyl, 5-methyl-                                                                          1.1    g                                                1,2,4-triazolium-3-thiolate                                                    2-mercapto-5-n.heptyl-oxa-                                                                            150    mg                                               3,4-diazole                                                                    water to make          1      l                                                ______________________________________                                    

    ______________________________________                                         Neutralization solutions                                                                         A              B                                             ______________________________________                                         NaH.sub.2 PO.sub.4.2H.sub.2 O                                                                    40     g       40   g                                        Non-ionic surfactant                                                                             200    mg      200  mg                                       Sodium benzoate   1      g       1    g                                        Sodium sulphite   4      g       4    g                                        Turpinal 2NZ.sup.a)                                                                              1      g       1    g                                        2-methyl-3 (2H)-  0      g       10   mg                                       isothiazolone                                                                  water to make     1      l       1    l                                        ______________________________________                                    

The pH of these 2 neutralization solutions is 6.0 +/-0.10. a) Turpinal 2NZ is a trade name of Henkel, Belgium for phosphonic acid, (1-hydroxyethylidene) bis-, disodium salt

    ______________________________________                                         Dampening solution                                                             ______________________________________                                         water                  880    ml                                               citric acid            6      g                                                boric acid             8.4    g                                                sodium sulphate anh.   25     g                                                ethyleneglycol         100    g                                                colloidal silica       28     g                                                ______________________________________                                    

The imaging sheets were identically exposed and developed with the above described alkaline activating solution for 20 s at 25° C., subsequently neutralized at room temperature with the neutralization solution A described above and dried. Without refreshing the processing solutions printing plates were so prepared in a camera platemaker A B DICK 1745, at an amount of 6.0 m² a day for a period of 68 days.

In a similar way printing plates were prepared by substituting the neutralization solution A by the neutralization solutions B.

The printing plates thus prepared at the first day and at different days were mounted on an offset printing machine AB DICK 1745, marketed by AB Dick Co, USA, equipped with a Aquamatic dampening system, marketed by AB Dick Co, USA, and were printed identical conditions. The above mentioned dampening solution ed at a 5% concentration for fountain solution. The ink used was ABD 1020, marketed by AB Dick Co, USA.

Evaluation:

The effect of the exhaustion of the neutralization solutions in function of their composition on the number of copies that had to be disposed of at the start of the printing process because of ink acceptance in the non-printing areas is given in table 2.

                  TABLE 2                                                          ______________________________________                                         Neutralisation solution                                                                             A     B                                                   ______________________________________                                         1 day                20    20                                                  11 days              15    15                                                  24 days              15    15                                                  33 days              20    15                                                  43 days              15    15                                                  50 days              20    15                                                  61 days              35    20                                                  68 days              50    20                                                  ______________________________________                                    

This number is low when the freshly prepared neutralization solutions A or B were used. However, after using these neutralization solutions for more than 50 days this number remains low when using the neutralization solutions B (solution according to the invention) but becomes unacceptable high when using the neutralization solution A (comparative solution). 

I claim:
 1. A method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of:image-wise exposing an imaging element comprising on a support a photosensitive layer including a silver halide emulsion and an image receiving layer containing physical development nuclei, developing said imaging element in the presence of at least one developing agent and at least one silver halide solvent using an alkaline processing liquid and neutralizing a thus obtained developed imaging element using a neutralization solution having a pH between 4 and 10.5 and comprising a buffer characterized in that said neutralization solution comprises 2-methyl-3(2H)-isothiazolone.
 2. A method according to claim 1 wherein the amount of said 2-methyl-3(2H)-isothiazolone ranges between 1 mg/l and 100 mg/l.
 3. A method according to claim 1 wherein said neutralization solution comprises a hydrophobizing agent.
 4. A method according to claim 1 wherein said neutralization solution further comprises a sequestering compound.
 5. A method according to claim 1 wherein said neutralization solution further comprises an agent having antioxidation activity.
 6. A method according to claim 1 wherein said neutralization solution comprises a monosodium and/or monopotassium dihydrogen phosphate.
 7. A method according to claim 1 wherein said neutralization solution further comprises an amine.
 8. A method according to claim 7 wherein said amine corresponds to the following formula: NR¹ R² R³ wherein R¹ represents a C₁ -C₁₂ hydrocarbon group containing a hydrophilic group and R² and R³ each independently represents a hydrogen, a C₁ -C₁₂ hydrocarbon group or R² and R³ together with the atoms to which they are attached complete a 5- or 6- membered ring, with the proviso that at least 1 of the groups represented by R² and R³ is hydrogen or an aliphatic group.
 9. A method according to claim 8 wherein said amine corresponds to the following formula: ##STR4## wherein X and X' independently represent hydrogen, a hydroxyl group or an amino group, l and m represent 0 or integers of 1 or more and n represents an integer of 1 or more.
 10. A method according to claim 1 wherein said neutralization solution is a concentrated solution which has to be diluted with water to obtain a ready-to-use neutralization solution. 